The present invention relates to organic electroluminescence (EL) devices, more particularly to electroluminescence devices comprising red light emitting materials using bissalicylaldiminato Schiff Bases and their metal complexes for thin-film type organic electroluminescence devices.
In 1960s, many trials for the preparation of organic electroluminescence devices were reported by using conjugated materials generally having fused aromatic rings (U.S. Pat. No. 3,172,862, issued 1965; U.S. Pat. No. 3,173,050, issued 1965). However, the efficiencies and lifetimes of these organic EL devices were much lower than those obtained from inorganic systems, therefore research activities were mainly focused on inorganic materials.
The reason for the low luminance of the early organic EL devices was the highly resistive EL medium, which prevented efficient injection of carriers into the light-emitting layer. Tang and VanSlyke solved this problem successfully in the 1980s (Tang and VanSlyke, Appl. Phys. Lett., 1987, 51, 913), improving significantly the performance of organic EL devices by using a two-thin-layer device with a hole-transporting layer of an organic substance laminated on an organic emitting layer. This work revived the research on organic EL devices, and resulted in the development of a new generation of light-emitting diodes with organic dyes.
Since then, many efforts have been made to further improve the properties of such EL devices, such as efficiency, stability and color purity (U.S. Pat. Nos. 5,141,671; 4,539,507; 6,020,078; 5,935,720; 5,972,247; 5,593,788; 4,885,211; 5,059,862; 5,104,740; 5,069,975; 5,126,214; 5,389,444; 6,165,383; 6,245,449; Chen et al, Macromol. Symp., 1997, 125, 1; Segura, Acta. Polym., 1998, 49, 319; Mitschke and Bauerle, J. Mater. Chem., 2000, 10, 1471). Among these, one of the most convenient and useful methods is to dope a strong emitting material into a host material to form a guest-host system. Thus, in principle, an organic EL device with good efficiency, high stability, as well as the desired color with proper chromaticity can be obtained by doping different strongly emitting materials into a host material such as tri-(8-hydroxyquinolinato)aluminum (AlQ3) to meet the requirement of practical applications.
In such a system, as a general rule the energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of a host material should be larger than that of the doped guest material to allow efficient energy transfer from the host to guest. By using this method, different colors can be achieved to meet the requirement for full-color applications.
It is generally understood that RGB (red-green-blue) materials with good color purity and high efficiency are essentially required for full-color applications. Extensive studies in the past decade have produced efficient blue and green materials which meet the requirement of commercial OLED applications. However, satisfactory red materials with good color purity, high efficiency and good stability are still lacking.
Although many red fluorescent dyes have been tested (Chen et al, Thin solid Films 2000, 363, 327; Chen et al, J. Phys. D. Appl. Phys. 2001, 34, 30; Picciolo et al, Appl. Phys. Lett. 2001, 78, 2378) and red phosphorescent dyes have recently been introduced (O""Brien et al, Appl. Phys. Lett. 1999, 74, 442), there is still an acute need for further improvement. For example, the color purity and efficiency of 4-(dicyano methylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), a current state-of-the-art red dopant, are still not satisfactory (see Chen et al, Thin solid Films 2000, 363, 327 and Chen et al, J. Phys. D. Appl. Phys. 2001, 34, 30).
Although rare-earth complexes have good color purity, their efficiency and chemical stability fall short of the requirements for commercial applications. Significant advances have recently been achieved using triplet emitters to obtain very efficient red devices. However, due to the long lifetime of triplet states, the density of triplet states can be easily saturated for high brightness applications. As a result, the efficiency of the triplet red emitters decreases rapidly with increasing brightness. This poses a serious problem for applications that require high excitation density such as in a passive dot-matrix display. Thus, new red dyes with high emission quantum yield, saturated red emission, and high stability are still much in demand.
For red-emitting materials, in general, two kinds of compounds are potentially good candidates: (1) compounds with large fused homo-aromatic rings; (2) intramolecular charge transfer (ICT) compounds with electron donating (D) groups and electron withdrawing groups (A) linked by conjugated structure. Since the fused homo-aromatic compounds with a large conjugated structure, especially those which emit in red, are often oxidised easily by singlet oxygen at ambient condition, this series of compounds is not expected to be suitable dopants unless the devices are used under dark conditions or without oxygen. Compared with the fused homo-aromatic compounds, ICT compounds have the following advantages:
[i] their emission wavelengths can easily be tuned by changing substituents to get different colors;
[ii] their molecular structures are relatively easy to modify for desired properties;
[iii] their Stokes shifts are generally large to prevent self-re-absorption efficiently, especially in the solid state;
[iv] they are chemically stable, and are not easily oxidised by singlet oxygen.
Given these advantages, intramolecular charge transfer (ICT) compounds are considered to be potential candidates for use as red-emitters. In particular, donor-acceptor-donor (D-A-D) type ICT compounds are useful as red-emitting materials for organic LEDs. The rationale is that D-A-D type ICT compounds often show desirable properties, aside from the general features of typical D-A type ICT compounds mentioned above. It is generally considered, albeit not well proven, that the fluorescence quantum yield of a D-A-D type ICT compound is higher than that of a D-A ICT compound with the same donor/acceptor moiety and conjugate structure. Moreover, symmetric D-A-D ICT compounds generally show better thermal stability and higher Tg than their D-A counterparts.
Salicylaldiminato Schiff bases are classical ligands for complexation with transition metals such as Cu2+, Ni2+, Co2+, Zn2+, Mn2+, Fe2+ etc. Many salicylaldiminato Schiff bases and their metal complexes have been prepared and are well documented. Recently, salicylaldiminato Schiff bases with intramolecular charge transfer property based on diaminomaleonitrile and p-diethylaminosalicylaldhyde have been used as DNA cleavage and non-linear optical (NLO) materials (Lacroix et al, I. Chem. Mater. 1996, 8, 541; Bella et al, J. Am. Chem. Soc. 1997, 119, 9550). However, these materials have not been tested or used in organic EL devices.
The object of the present invention is to provide new organic electroluminescence devices which address some of the problems discussed above. In particular, it is an object of the invention to provide organic EL devices with pure saturated red emission, and with a narrow emission band.
The present invention provides an organic electroluminescence device comprising an anode, a cathode, a luminescent layer, at least one hole-transporting layer disposed between the said anode and said luminescent layer, at least one electron-transporting layer disposed between said cathode and said luminescent layer, and a substrate present on either the anode or cathode, wherein the luminescent layer comprises a compound of formula [I]: 
wherein R5-R8 are independently selected from the group consisting of hydrogen and halogen atoms, hydroxy, cyano, nitro, mercapto, carbonyl and sulfone groups, and optionally substituted alkyl, haloalkyl, hydroxyalkyl, aryl, alkoxy, aryloxy, alkylamino, arylamino, alkylarylamino, alkylthio, arylthio, ester, siloxy, cyclic hydrocarbon or heterocyclic groups;
either R9 and R11 are both hydrogen and R1 and R2 are independently selected from the group consisting of hydrogen and halogen atoms, hydroxy, cyano, nitro, mercapto, carbonyl and sulfone groups, and optionally substituted alkyl, haloalkyl, hydroxyalkyl, aryl, alkoxy, aryloxy, alkylamino, arylamino, alkylarylamino, alkylthio, arylthio, ester, siloxy, cyclic hydrocarbon or heterocyclic groups, or R1 and R9, and R2 and R11 each, together with the interjacent carbon and nitrogen atoms, form an optionally substituted ring having between 5 and 7 ring atoms; and
either R10 and R12 are both hydrogen and R3 and R4 are independently selected from the group consisting of hydrogen and halogen atoms, hydroxy, cyano, nitro, mercapto, carbonyl and sulfone groups, and optionally substituted alkyl, haloalkyl, hydroxyalkyl, aryl, alkoxy, aryloxy, alkylamino, arylamino, alkylarylamino, alkylthio, arylthio, ester, siloxy, cyclic hydrocarbon or heterocyclic groups, or R3 and R12, and R4 and R10 each, together with the interjacent carbon and nitrogen atoms, form an optionally substituted ring having between 5 and 7 ring atoms; or a metal complex equivalent thereof.
The invention also provides the use of a compound of formula [I], as described above, as a luminescent material in an electroluminescence device. There is further provided a method of using a compound of formula [I] in an electroluminescence device, the method comprising providing a compound of formula [I], and incorporating said compound as a luminescent material within an electroluminescence device which further comprises an anode, a cathode, a hole-transporting layer and an electron-transporting layer.
The invention further provides a compound of formula [I], as described above, with the proviso that the compound of formula [I] is not 2,3-bis-[[(2-hydroxy-4-diethylamino)phenyl](methylene)]-amino]-2-butenedinitrile or the copper, nickel or zinc complex thereof.
The electroluminescence devices of the intention have excellent color chromaticity co-ordinates and good efficiency, and can emit in red. Furthermore, the materials provided by the present invention are eased to synthesise and purify in high yields, thereby being very attractive from an economic perspective, and, when the materials are present as dopants within a host material, have electroluminescence spectra which do not change with doping concentration.